Formation of Doubly Charged Argon Ions, Ar²⁺, from Long-Lived Highly Excited Argon Ions, Ar^+*, Colliding with Ar and N₂ Gases

Abstract

Formation of Ar²⁺ from long-lived highly excited Ar^+* colliding with Ar and N₂ gases is studied by means of a tandem mass spectrometer. The tandem mass spectrometer used consists of two mass analyzers connected in series and a collision chamber located in between. The collision chamber is electrically floated and can be set at a desired potential, so that one can identify the fast ions(resulting from the primary ions)and the slow ions(secondary ions)in the mass spectra taken by the second mass analyzer. When the first mass analyzer is tuned to Ar⁺, peaks corresponding Ar²⁺ appear in the second mass spectra. From the analysis of variation of mass positions and heights of these peaks with the change of the potential and pressure of the collision chamber, the Ar²⁺ is concluded to result from the primary Ar⁺ in collision with gas molecules and wall surface. From the threshold behavior of the product Ar²⁺ with the electron energy in the ion source, three sets of long-lived highly excited Ar^+* states(Rydberg states)are found to be responsible for this process. They are 3s²3p⁴(³P)n1, 3s²3p⁴(¹D₂)n'1 and 3s²3p⁴(¹S₀)n″1 converging to Ar²⁺³P_2.1.0(43.38, 43.51, 43.57 eV), ¹D₂(45.11 eV)and ¹So(47.50 eV), respectively. Their fractional ratio in the primary Ar⁺ beam is determined as 3.0:1.0:1.2 which is close to that of multiplicities of the states concerned. The autoionization mechanism reported by other investigators to be responsible for the formation of Ar²⁺ in Aston band or tandem mass spectra is found to be negligible. The cross sections of formation of Ar²⁺ from Ar⁺ colliding with Ar and N2 increase in proportion to the1.15th power of the collision energy in the range from 750 eV to 2.5 keV. At the collision energy of 1.0 keV, they are 2.0×10^-20/Fcm² for Ar target and 6.6×10^-20/Fcm² for N₂ target, where the fractional density of Ar^+* is estimated to be0 .7×10^-4≤F≤1.5×10^-4.